1. Field of the Invention
The present invention relates to optical communication systems, and more particularly to an apparatus and method for tuning an optical interferometer.
2. Description of the Prior Art
Optical interferometers made using optical fiber or silica waveguide are not stable devices. They are particularly susceptible to temperature variations. As the temperature proximate the optical interferometer changes, the path length of the optical fibers or silica waveguide making up its legs likewise change. This results in a change in the interference pattern created by the optical interferometer. To compensate, the optical interferometer must be tuned continuously.
An apparatus and method for tuning an optical interferometer is known in the art. An example of such an apparatus is described in an article by Eric A. Swanson, Jeffrey C. Livas and Roy S. Bondurant, entitled xe2x80x9cHigh Sensitivity Optically Preamplified Direct Detection DPSK Receiver With Active Delay-Line Stabilization,xe2x80x9d in IEEE Photonics Technology Letters, Vol. 6, No. 2, February 1994. This article describes an optical communication system that modulates digital information onto transmitted light using differential phase shift keying (DPSK) and then demodulates this information using an actively tuned unbalanced Mach-Zehnder optical interferometer that is tuned using an apparatus and a method known in the art. The unbalanced Mach-Zehnder optical interferometer has an additional optical path length in one leg that provides a propagation delay duration of one data bit. The imbalance in the Mach-Zehnder optical interferometer enables light in one data bit to be optically interfered with light in the data bit immediately following this data bit. The relative state of optical phase between these two DPSK data bits determines in which of the two output legs of the interferometer light is produced provided that the unbalanced Mach-Zehnder optical interferometer is properly tuned within a fraction of a wavelength of the light. Light produced from one leg constitutes digital xe2x80x9conesxe2x80x9d while light produced in the other leg constitutes digital xe2x80x9czerosxe2x80x9d in the transmitted digital information signal. This article also describes an apparatus and a method for using optical amplification to improve receiver sensitivity that utilizes a doped optical fiber amplifier to boost the signal level and a Fabry-Perot narrow band filter to remove the out-of-band amplified spontaneous emission (ASE) introduced by the fiber amplifier.
The apparatus described in the article includes a laser and a phase modulator for producing an optical DPSK signal at a preselected wavelength, a 10 GHz tunable fiber Fabry-Perot filter and an automatic controller for dithering the pass band wavelength of the filter so as to keep the peak of the filter at the optical signal wavelength, a tunable unbalanced Mach-Zehnder optical interferometer, a dual balanced detector and a feedback electronic circuit coupling the signal developed across one detector of the balanced detector to one leg of the Mach-Zehnder interferometer.
The article describes two different approaches to provide tuning of the optical path length in the unbalanced Mach-Zehnder optical interferometer. In the first approach, the interferometer is made of optical fiber and one leg of the interferometer is wrapped around a piezoelectric transducer (PZT) that enables an electronic signal to stretch the fiber, thereby increasing the optical path length. In the second approach, the interferometer is made of a silica integrated optical waveguide with an integral thermal heater that enables an electronic signal to increase the temperature of one leg of the interferometer, thereby increasing the optical path length.
The known art for tuning the Mach-Zehnder presented in this article uses a small electronic dither signal applied to the actively tuned optical path length to provide a feedback signal for the electronic controller enabling proper adjustment of the optical path length. Electronic synchronous detection techniques on this dither signal are used to provide the appropriate corrections to the optical path length, enabling the error in tuning to be below an acceptable level.
The prior art approaches for actively tuning an optical interferometer have several disadvantages. First, they introduce an undesired optical intensity dither on top of the original optical communication signal that is intended to be extracted. This dither arising from the intentional dither of the optical path length is actually a source of noise that degrades the fidelity of the original communications signal. Second, the approach using the heater to perform the dither and tuning is restricted to relatively low frequencies of dither due to the relatively large thermal time constant of the heater.
Third, the approaches introduce a small dithering variation in the interference state delivered at the output of the Mach-Zehnder interferometer. This precludes the use of the interferometer in applications where an absolute quiet state of interference must be maintained.
What is needed, therefore, is an apparatus for actively tuning an optical interferometer that allows the interferometer to be quietly maintained at a relatively constant optical path length.
In addition, it is desirable to actively tune the optical interferometer without introducing any dither in its optical path length and accommodate effects of changes in temperature or wavelength.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provide generally, in a first aspect, an apparatus for using an optical signal to actively tune an optical interferometer by dithering the wavelength of the input light so as to create the same optical feedback signals for tuning the optical interferometer as used in the prior art without introducing dither to the optical path length of the leg of the optical interferometer. The apparatus comprises means for generating a dithering signal, means for applying a portion of said dithering signal to the optical signal so as to provide an optical signal having a varying wavelength in accordance with said dithering signal, an optical interferometer having one optical path length that is electronically tunable, photodetector means on each of the two optical interferometer outputs for developing an electronic feedback signal that includes a portion representative of the dithering signal, means for synchronously extracting the dithering signal from the electronic signal, and means responsive to the extracted dithering signal and responsive to appropriately electronically tune the optical path length in the interferometer. More particularly, the optical interferometer is responsive to the optical signal having a varying wavelength in the same way that the optical interferometer is responsive to an optical path length dithering adjustment using the known prior art in that the optical interferometer is operative to develop a first interference pattern where the optical path length or wavelength is a prescribed value and is operative to develop a different interference pattern when the optical path length or wavelength is changed. Also, the photodetector means is responsive to the optical interference pattern and is operative to develop the electronic feedback signal when the first interference pattern is not present.
In another aspect, the present invention provides a method of using an optical signal for tuning an optical interferometer by adjusting the optical path length of one of its optical paths to produce a preselected interference pattern, comprising the steps of generating a dithering signal, applying a portion of the dithering signal to the optical signal to vary its wavelength, applying the varying wavelength optical signal to the optical interferometer so as to change the interference pattern it produces, developing an electronic signal representative of the changed interference pattern, applying a portion of the dithering signal and the electronic signal to a synchronous detector to create a drive signal, and applying the drive signal to the one optical path in a manner to adjust its optical path length such that is the preselected interference pattern is formed, thereby tuning the optical interferometer.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description.